Summary: The goal of this project is to understand the molecular basis of lymphocyte differentiation from na?ve to memory, and immunological memory. As the first step toward this goal, we have analyzed gene expression profiles of human naive and memory CD4+ T cells, and identified genes that are differentially expressed in one population over the other. Human CD4+ naive and memory T cells are phenotypically well defined subsets, i.e. naive CD4+ T cells expressing CD45RA and memory CD4+ T cells expressing CD45R0. We used immunomagnetic separation isolating large number of highly pure naive and memory cells from peripheral blood. Messenger RNA was isolated from naive and memory T cells and used for gene expression analysis. Gene expression profile and differentially expressed genes were analyzed by cDNA microarray. Approximately half of the estimated total human genes (45,000) were analyzed and found that na?ve and memory CD4+ T cells expressed about 15-20% of total genes. The levels of expression about 85% of the expressed genes are similar between naive and memory cells. Using an expression ratio of 2 as criteria of differential expression, we found that about 300 genes (4% of total expressed genes) that are differentially expressed in na?ve CD4+ T cells and about 740 genes (9% of total expressed genes) differentially expressed in memory CD4+ T cells. Among these genes, only 5-10% of these differentially expressed known genes and the majority of them are ESTs. Currently, we are characterizing these differentially expressed genes by Northern, in situ hybridization, and sequencing. - Naive and memory T cells, cDNA microarray, lymphocyte activation, anergy